Method and a network element for transferring data packets in a teletransmission network

ABSTRACT

In a method of transferring data packets between a plurality of network elements ( 14; 28 ) in a teletransmission network ( 1 ), at least some data packets are transferred at a bit rate which depends on available network capacity. When arriving at a network element, packets are positioned in a plurality of input queues ( 33; 37 ). Then, the packets are transferred from the input queues ( 33; 37 ) to at least one output queue ( 38 ) at a bit rate which is regulated by a factor (λ) which depends on the filling degree of the output queue.

The invention concerns a method of transferring data packets between aplurality of network elements in a teletransmission network, wherein atleast some data packets are transferred at a bit rate which depends onavailable network capacity. The packets, when arriving at a networkelement, are positioned in a plurality of input queues and are thentransferred to at least one output queue. The invention moreoverconcerns a network element for use in such a teletransmission network.

A plurality of methods is known for the transfer of data packets betweena plurality of network elements in a teletransmission network. Anexample of such a method is ATM (Asynchronous Transfer Mode). ATM is apacket technology which transfers data packets of a fixed, short length.In the ATM the data packets are called cells, and each cell consists of53 bytes, of which 48 are used for information transfer proper, while 5bytes are so-called header information used for controlling thetransmission.

One of the advantages of the ATM and other corresponding technologies isthat network capacity is occupied only in relation to the amounts ofdata which are actually transferred, as the system uses so-calledvirtual connections which are set up by transmitting a set-up requestthrough the network to the desired destination.

The physical network itself, in which the data packets are transmitted,may e.g. be a fibre-optical network using PDH (Plesiochronous DigitalHierarchy) or SDH (Synchronous Digital Hierarchy). Also, a variety ofphysical media or protocols may be included within the sameinfrastructure.

A virtual ATM connection passes a plurality of network elements en routein the network from source to destination. The task of an ATM networkelement is to determine on the basis of the header information of areceived ATM cell to which network element output the cell is to bepassed, and then to perform this operation while updating the headerinformation of the cell.

As ATM cells to a specific output on an ATM network element may comefrom many different inputs of the exchange, it may happen that the totalinstantaneous rate of these cells may exceed the capacity of the output.It is known e.g. from European Patent Application EP 680 180 to solvethis problem by means of large buffers or queues capable ofaccommodating a brief overload of cells in the network element.

As it is precisely one of the advantages of the ATM and othercorresponding systems that network capacity is occupied only in relationto the amounts of data which are actually transferred, and as data aretypically transferred in “bundles”, the network load will vary strongly.At some times the capacity is almost utilized fully, while at othertimes there will be a great surplus capacity. This opens up thepossibility of offering a service called ABR (Available Bit Rate) whichmay be used for data insensitive to delays. These data are thentransferred only in case of available capacity. If the capacity issuddenly occupied by data of higher priority (non-ABR data), thetransmission is suspended or limited until there is available capacityagain.

European Patent Application EP 678 997 discloses an ATM system capableof also handling ABR. A network element in this system has an input andan output module for each line in the network connected to the networkelement. The input modules include special storages or buffers which arespecially intended for ABR traffic, a buffer or queue being provided foreach output module. The system allocates capacity to these buffers onlywhen all other traffic has been handled, and the transmission isinterrupted again if the capacity is again needed for traffic of higherpriority. When an input module has ABR traffic to an output module, anABR request is transmitted to the output module, where it is registeredin a resource allocator. When the output module has available capacity,the resource allocator transmits a signal to the input module concerned,and the ABR traffic is then transferred to the output module and furtheron the associated line in the network.

Although this solution is capable of handling ABR traffic in an ATMsystem, the internal communications in the network element are rathercomplex and thus expensive to implement. Further, as a result ofinexpedient internal regulation of the ABR traffic, the buffers orqueues used must be very long to avoid data losses, since arriving ABRcells are to be stored in these queues until the output module concernedhas available capacity. Moreover, the system does not permit fairdistribution of the available output module capacity to the arriving ABRtraffic.

The international patent application WO 94/14266 discloses a system inwhich input queues as well as output queues are provided in a networkelement for e.g. ATM traffic, and in which transfer of data packets froman input queue to an output queue is blocked when the filling degree ofthe output queue exceeds a threshold value. The output queues are herebyprotected against overfilling which would result in loss of data. Thissystem, however, is vitiated by the serious drawback that transfer ofdata packets from a given input queue to a given output queue can onlybe entirely allowed or entirely blocked, and moreover the system cannotdistinguish between e.g. ABR traffic and ordinary traffic of higherpriority. This is a problem, because it is exclusively the filling of anoutput queue which determines whether one or more of the input queuesare closed. Thus, it is not possible to give the arriving data packetsdifferent priorities, and the system involves the risk that an outputqueue is filled with ABR packets and is therefore closed to moreimportant traffic. Furthermore, the circumstance that transfer of datapackets from an input queue to an output queue can only be entirelyallowed or entirely blocked, means that the data processing in thenetwork element will be uneven.

Further, the system is arranged so as to stop transfer of data to anoutput queue when the filling degree of said queue exceeds anestablished threshold value, e.g. when it is half full; but this meansthat it is not possible to utilize the capacity of the queue fully as itwill always have a plurality of empty locations. The system musttherefore be provided with excessively large output queues. The sameapplies to the input queues as these are attached to the incomingphysical connections, and data packets to be distributed to severaldifferent output connections may arrive on one incoming connection. Ifan input queue is closed because the “first” data packet in the queue isfor an overfilled output queue, also the subsequent data packets will beshut off even though these are for other output queues which are notover-filled. In addition to greater input queues, this means that othertraffic of higher priority may be unduly delayed.

The international patent application WO 95/15637, which addresses asomewhat different problem, discloses attaching an input queue to eachvirtual connection in a network element in a teletransmission network.This allows for the situation just described, where data packets for anoverfilled output queue shut off data packets for other output queues;but this does not remedy the other described problems.

Accordingly, an object of the invention is to provide a method of thetype stated in the opening paragraph which has a simpler and moreefficient internal traffic regulation, and which is capable of reducingthe length of the buffers or queues used. Further, the method mustpermit a fair distribution of the available capacity.

This is achieved according to the invention in that the data packetstransferred at a bit rate depending on available network capacity aretransferred from the input queues to at least one output queue providedspecifically for those packets at a bit rate which is regulated inseveral steps by a factor which depends on the filling degree of theoutput queue.

When the bit rate from the input queues to the output queue is regulatedin several steps in dependence on the filling degree of the outputqueue, a more even filling of the output queue is obtained, since,usually, the transfer is not interrupted completely. As, when an outputqueue is nearly full, data packets are still transferred from the inputqueues, merely at a lower bit rate, it will still be possible to havedifferent priority for the different input queues and thus maintain afair distribution of the available capacity, as mentioned below.Furthermore, a relatively short output queue may be used, as theregulation ensures that the number of data packets in the queue will besubstantially constant. Further, the system will always be ready totransmit ABR traffic when capacity becomes available, because it willnot have to first fetch the data packets from the input queues.

When, several output queues are used and the bit rate is regulatedseparately for each output queue, it is moreover possible to allow forvariations in available capacity between the individual outputs.Typically, an output queue is used for each outgoing line from thenetwork element.

The most expedient regulation is provided when the said factor isrelatively stable, and this is achieved by currently calculating thefactor on the basis of the mean value of the filling degree during apreceding time span. This is in contrast to WO 94/14266 where, instead,sampling of the filling degree takes place each time a data packet istransferred, which results in a more unstable regulation.

A further improvement of the method is achieved by moreover regulatingthe bit rate by a factor indicative of a mutual weighting of the inputqueues. As mentioned, this ensures a more fair distribution of thecapacity which is available, as the proportion between the transmissionrates of a plurality of input queues supplying data to the same outputqueue constantly has a mutual proportion determined by the allocatedweights. This mutual proportion is maintained unchanged when the overallcapacity of an output queue is reduced by means of the first factor as aconsequence of the queue being full.

When e.g. an ATM system is involved, the data packets or the cells aretransmitted via a large number of virtual connections in the network,and then a particularly flexible regulation is achieved, by using aninput queue for each virtual connection arriving at the network elementconcerned, because the individual virtual connections may then beweighted separately without considering whether they perhaps arrive atthe network element on the same physical line. This takes place in thateach input queue has its own weight factor.

In case of sudden changes in the data amount it may occur that theoutput queue becomes full, without the regulation factor having beenable to regulate the bit rate from the input queues down, since, asmentioned, averaging is performed over a period of time. This is avoidedby then interrupting the transfer of data packets from the input queuescompletely. Data losses are hereby avoided in this situation.

In an expedient embodiment of the invention, the transmission network isan ATM network, and the said data packets, transferred at a bit ratedepending on available capacity in the network, are formed by ABRtraffic.

As mentioned, the invention moreover concerns a network element for usein such a transmission network. When the element comprises an outputqueue specifically provided for packets transferred at a bit ratedepending on available network capacity and means for regulating the bitrate in several steps depending on the filling degree of this queue, theadvantages described above are achieved. The invention may concernexpedient embodiments of the element corresponding to the matterdescribed above.

Such a network element typically comprises a switching unit, a pluralityof input modules and a plurality of output modules, and in that case itis expedient to position the input queues on the input modules, becausethe large number will result in an unduly complex control if the inputqueues are provided in the switching unit itself, as is usually thecase.

The output queue or queues may be positioned in the switching unit or,when an output queue is provided for each output module, be positionedon these.

The invention will now be explained more fully below with reference tothe drawing, in which

FIG. 1 shows a network element in which the invention may be applied,

FIG. 2 shows a network element for use in the network of FIG. 1,

FIG. 3 shows an example of an embodiment of a network element,

FIG. 4 shows a model of ABR queues in the element of FIG. 3,

FIG. 5 shows an embodiment of a network element according to theinvention,

FIG. 6 shows feedback in the element of FIG. 5,

FIG. 7 shows another embodiment of a network element according to theinvention, and

FIG. 8 shows feedback in the element of FIG. 7.

FIG. 1 shows a simplified example of how an ATM network 1, in which theinvention may be applied, may be designed. The network sets upconnections between a plurality of ATM terminals 2-4 (users or ATMsubscribers), and it consists of a plurality of network elements 5-7 aswell as a plurality of transmission lines 8-13 which connect theterminals with the network elements and these with each other.

ATM is a packet technology transferring traffic between ATM terminals inshort packets or cells, each of which is of 53 bytes, of which 48 bytesare used for information proper, while 5 bytes are used for so-calledheader information used inter alia for controlling the transmission. Thephysical transmission medium providing for the transmission of the datapackets or the ATM cells along the transmission lines 8-13, may e.g. beoptical fibres using PDH (Plesiochronous Digital Hierarchy) or SDH(Synchronous Digital Hierarchy), and the system may e.g. be used forwideband ISDN (Integrated Services Digital Network).

The task of the ATM network is thus just to transfer ATM cells from onepoint in the network to another, which, as mentioned, is controlled bythe overhead information. The ATM traffic to be transferred by thenetwork may have a constant or variable bit rate, and owing to thelatter in particular the transmission takes place by means of virtualconnections, which thus just occupy transmission capacity in relation tothe amount of data transferred in reality. A virtual connection is setup by transmitting information from the transmission point to thenetwork elements which have to be passed to arrive at the destination,concerning which way the elements are to transmit the ATM cellsassociated with the connection. These may be identified by means oftheir overhead information.

As mentioned, the network 1 in FIG. 1 is simplified, it being just shownwith three network elements 5-7 which are interconnected by threetransmission lines 11-13. In practice, an ATM network will be far morecomprehensive and consist of a large number of network elements whichare interconnected by a large number of transmission lines. It should bestressed that a large number of virtual connections may be transferredon each transmission line. In FIG. 1, e.g. a virtual connection fromterminal 3 to terminal 4 will proceed along the transmission lines 9, 13and 10 and en route pass the network elements 6 and 7.

FIG. 2 shows a network element 14 which is connected to fourtransmission lines 15-18. ATM cells associated with a virtual connectionmay e.g. arrive at the network element 14 on the line 15, and thenetwork element is then to decide on the basis of the overheadinformation of the cells on which line the cells are to be transmitted.As appears from the arrows on the transmission lines, these arebidirectional. Accordingly, an input module 19-22 and an output module23-26 are provided for each line. The actual switching and the controlthereof take place in the switching unit 27.

The described structure of a network and a network element,respectively, is the same, no matter whether ABR traffic occurs in thenetwork or not. ABR traffic is traffic which is insensitive to delays inthe transmission, i.e. traffic where it is without importance when itarrives at the destination, as long as it arrives safely. Typically,this traffic is buffered or queued in the input modules of the networkelements until capacity is available on the relevant output line, andthen it is transmitted and hereby utilizes surplus capacity in thenetwork. Examples of applications capable of utilizing ABR are filetransfers, electronic mail, distributed computation power, interactiveservices, etc.

The rest of the traffic, here called non-ABR traffic, must have priorityover the ABR traffic, as, otherwise, the ABR traffic might occupy theentire capacity. Existing priority levels in the non-ABR traffic are notaffected by whether ABR traffic occurs or not.

FIG. 3 shows a general network element 28 intended to handle non-ABRtraffic as well as ABR traffic according to the invention. The elementhas a plurality of ingoing lines 29 and a plurality of outgoing lines30. As described before, the ingoing and the outgoing lines will usuallybe associated in pairs corresponding to the physical transmission lines.Each of the outgoing lines 30 has an associated queue module 31 in whichATM cells (both ABR and non-ABR) to be transmitted along the output lineconcerned are temporarily queued. As will be seen, a plurality of queues32 is provided for the non-ABR traffic and a plurality of queues 33 isprovided for the ABR traffic. In the prior art, there is usually one ABRqueue for each outgoing line, but, as will be seen, a plurality of ABRqueues is provided here on each queue module. The reason is that oneinput queue is used here for each virtual connection which passes thenetwork element 28. The number of queues will thus vary with the numberof virtual connections set up, and typically there will be many queuesfor each outgoing line.

FIG. 4 shows a simple model of how the ABR queues can operate. Thetransmission rate r_(c) of the queue belonging to the virtual connectionc, is given by: $\begin{matrix}{r_{c} = \left\{ {\begin{matrix}{{\min \quad \left\{ {{W_{c} \cdot \lambda},{ri}_{c}} \right\}},} & {{{if}\quad Q\quad l_{c}} = 0} \\{{W_{c} \cdot \lambda},} & {{{if}\quad Q\quad l_{c}} > 0}\end{matrix},} \right.} & (1)\end{matrix}$

wherein ri_(c) is the incoming cell rate of the connection c, W_(c) isthe relative weight of the connection c, Ql_(c) is the length of thevirtual queue for the connection c, and λ is a value selected such that

B _(o) ≧Σr _(c),  (2)

where B_(o) is the available rate of the output line o. If the formula(2) is satisfied for infinitely high values of λ, the virtual queues areempty, and the formula (1) is reduced to r_(c)=ri_(c). An ABR connectionmay be in three states, active when Ql_(c)>0, inactive when r_(c)=0, anda transitory state when Ql_(c)=0 and ri_(c)>0. The transitory stateoccurs when cells begin arriving at an empty queue, and this situationis without importance for the following.

It will be seen from the formula (1) that when cells are present in oneof the queues (the associated connection is active, i.e. Ql_(c)>0), thetransmission rate from this queue is given by the expression W_(c).λ,and it appears from the formula (2) that λ is a factor ensuring that thetotal transmission rate from the queues transferring cells to a givenoutput line does not exceed its available rate B_(o), which varies inresponse to the non-ABR traffic. When the capacity available for ABR onan output line is reduced, λ will thus have to be reduced for thisoutput line so that fewer cells are transferred from the connectedqueues. The transmission rate of all queues associated with a specificoutput is thus reduced by the same factor. It will be described morefully below how λ may be derived. The weight factor W_(c), on the otherhand, is determined individually for each virtual connection and thusfor each input queue and indicates the proportion between thetransmission rates of the individual queues. The combination of the twofactors ensures a fair distribution of the available capacity, as themutual proportion determined by W_(c) is maintained unchanged, also whenthe total transmission rate to a given output line is regulated up ordown by means of the factor λ.

In FIG. 3, the input queues 33 associated with the virtual connectionsare arranged on queue modules 31, there being one queue module for eachoutgoing line. The queue modules are therefore positioned in theswitching unit 27 itself (FIG. 2) or optionally with one queue modulepositioned in each output module. This solution, however, may be verycomplex, as the number of queues will be very large and the controlfunctions therefore extremely complicated. It will therefore beexpedient to place the input queues in the input modules instead, aseach input module then has queues for the virtual connections enteringthe associated input line.

This is shown in FIG. 5. The ingoing lines 29 and the outgoing lines 30correspond to FIG. 3. Here, the network element has a plurality of inputmodules (corresponding to the plurality of ingoing lines), of which theinput module 34 is shown. As will be known from FIG. 2, these areconnected to the switching unit 35 which has a plurality of queuemodules 36 corresponding to the plurality of outgoing lines. The non-ABRtraffic is processed entirely like in FIG. 3, but, as shown, now justpasses through the input modules to the queues 32 on the queue module.For the ABR traffic, on the other hand, there is now a plurality ofinput queues 37 on each input module 34, said plurality corresponding tothe plurality of virtual connections, as mentioned, which enter theassociated input line. Now just one ABR queue 38 for each output line ispresent on the queue modules 36, and this queue, which is called theoutput queue, collects ABR cells to the output originating from theinput queues whose virtual connection is to be fed to the outputconcerned. As will be described more fully below, the plurality of cellsqueued in this queue may be utilized for deriving the factor λ in that,if the queue is being filled, λ is reduced, thereby down-regulating therate at which the cells are transferred from the input queues to theoutput queue.

The necessary size of the input queues, each of which corresponds to avirtual ABR connection, as mentioned, will frequently be of e.g.1,000-50,000 cells, and, as mentioned, these queues should beimplemented on the input modules rather than in the switching unit, asthe requirement with respect to the rate of the storage implementing thequeues is 2R on the input modules and R(N+1) in the switching unit,where R is the bit rate and N is the number of output ports on theswitching unit. When the input queues are positioned on the inputmodules, the size of the ABR queue in the switching unit will be limitedto e.g. 200-500 cells. The feedback from the switching unit to the inputunits (i.e. λ) is to ensure that the queues positioned in the switchingunit will neither overflow nor be emptied, if ABR cells are queued for agiven output on one of the input modules.

The feedback information consists of a soft and a hard feedback. Thesoft feedback (λ) ensures fairness between the ABR connections andregulates the ABR traffic from the input modules. The hard feedbackprevents cell losses, if the ABR buffer in the switching unit should befilled-up completely. All input modules supplying cells to a givenoutput queue receive the same hard and soft feedback. The feedbackinformation is shown in FIG. 6, which corresponds to FIG. 5. The inputmodules 34 and the switching unit 35 with the queue modules 36 are thesame, and also the output modules 40 are shown. In the calculatingcircuit 41, the switching unit calculates the value of λ for the outputconcerned by using the length of the relevant ABR queue 38. The λ valuesare transferred as shown from the calculating circuit 41 to a pluralityof traffic adaptation circuits 42 in the input module 34 and thusconstitute the soft feedback. The feedback is just shown to one of thetraffic adaptation circuits 42 in the figure, but takes place of courseto all the circuits which are suppliers to the output queue 38. Each λvalue (i.e. for each output) is transferred each time T_(update) periodshave elapsed, and the soft feed-back thus on average transfersN/T_(update) values of λ in each cell period, where N is the number ofoutput ports on the switching unit. When the adaptation functions 42need to know the instantaneous permitted rate of a given connection, therate is calculated on the basis of the λ value of that connectionreceived last.

The actual calculation of λ in the circuit 41 is quite complicated ifthe calculation is to be accurate, and accordingly an approximatedmethod is used, which appears from the below formula for the calculationof λ_(o,t), i.e. λ for the output line o at the time t. As mentionedabove, and to save calculation power, λ_(o,t) is only calculated eachtime T_(update) periods have elapsed. As the expression n↑m for twointegers n and m is defined as (m mod n)=0, λ_(o,t) is given by:$\begin{matrix}{\lambda_{o,t} = \left\{ {\begin{matrix}{{f\quad \left( {\lambda_{o,{t - T_{update}}},{Q\quad l_{o,t}},{Q\quad l_{o,{t - T_{update}}}}} \right)},} & {t > {0\bigwedge\left. T_{update}\uparrow t \right.}} \\{\lambda_{o,{t - 1}},} & {\left( T_{update}\uparrow t \right)} \\{0,} & {t = 0}\end{matrix}.} \right.} & (3)\end{matrix}$

An example of a simple algorithm f( ) for a control loop to calculateλ_(o,t) is:

f(λ_(old) ,Ql,Ql _(old))=max{λ_(old) −F _(a)·(Ql−Ql _(old))−F_(b)·(Ql−Ml),0},  (4)

where

t is the time in cell periods in the switching unit

Ql_(o,t) is the queue length of the output queue of port o at time t

Ml is the queue length in an imaginary stable state with 100%utilization of B_(o),

F_(a) and F_(b) are constants, and

T_(update) is the calculation frequency of λ (number of cell periods).

The hard feedback 43 contains information on full ABR queues in theswitching unit. When an input module detects a full ABR queue in theswitching unit, it stops transmitting cells which are to be transferredto that queue. It should be noted that the hard feedback just preventscell loss in the switching unit, but does not guarantee fairness. If thesoft feedback and the adaptation function 42 were ideal, the hardfeedback would not be necessary. The hard feedback 43 is used when thesoft feedback cannot reduce the cell rates from the input modules fastenough.

The overall system ensures that no empty cells are transmitted to agiven port, if cells are queued for that port somewhere in the system.However, this does not apply if the ABR queue for a given output port inthe switching unit is empty, while ABR cells are queued for that port inthe input queues. This situation occurs when the soft feedback cannotincrease λ fast enough.

In general, the ATM network elements may be provided with both inputmodules capable of handling ABR according to the invention, and modulesnot capable of handling ABR. In that case, the actual switching unitmust be adapted for ABR and be capable of supplying the hard and thesoft feedback, and, of course, ABR connection can only be set up betweeninput and output modules which are capable of handling ABR.

An alternative to the solution shown in FIGS. 5 and 6 is shown in FIG.7, where queues are also positioned on the output modules. The inputmodules 34 and the switching unit 35 with the queue modules 36 are thesame as in FIG. 5. In addition, queues are positioned on the outputmodules 39. This is also called submultiplexing, and the queues on theoutput modules are called submultiplexing queues. The queue structure isthe same on the output modules 39 as in the switching unit 35, as theoutput queues have merely been moved from the switching unit to theoutput modules. The queues in the switching unit are now used for rateadaptation from the many input modules to an output module.

FIG. 8 shows the hard and the soft feedback in this situation. The ABRqueues which here constitute the “bottleneck” with respect to thedetermination of the λ value, are the queues on the output modules 39,and the circuit 41 is therefore provided here, and, like before, thesoft feedback is fed from there to the adaptation circuit 42 in theinput module 34. The calculation of λ can take place in the same manneras described before.

The risk of an ABR queue in the switching unit becoming full is the sameas in FIG. 6, and the hard feedback 43 is therefore still necessary toprevent cell loss in the switching unit. The function of the hardfeedback is therefore unchanged.

Because of the submultiplexing used, the buffers on the output moduleswill be greater than those in the switching unit, and since the cellrate to the output module is therefore lower than in the switching uniton average, the probability of the ABR queues in the output modulebecoming full is reduced. If this should happen nevertheless, thefeedback 44 will stop all cells to the output module, i.e. also to theother ABR queues, even though these are perhaps not full. This isacceptable, since the ABR queues on the output modules will only rarelybe full, as mentioned.

Although a preferred embodiment of the present invention has beendescribed and illustrated, the invention is not restricted to this, butmay be also be embodied in other ways within the scope of thesubject-matter defined in the following claims.

What is claimed is:
 1. A method of transferring data packets between aplurality of network elements in a teletransmission network, comprising:transferring at least some data packets at a bit rate which depends onavailable network capacity; positioning the at least some data packets,when arriving at a network element, in a plurality of input queues;transferring said at least some data packets from the input queues to atleast one output queue provided specifically for said at least somepackets at a bit rate which is regulated in several steps by a firstfactor which depends on the filling degree of said at least one outputqueue; completely interrupting transfer of said at least some datapackets from the input queues to said at least one output queueindependent of said regulation of the bit rate in response to the atleast one output queue reaching its full capacity.
 2. A method accordingto claim 1, characterized in that several output queues are providedspecifically for said at least some packets, and that the data packetsfrom each input queue are transferred to one of the output queues at abit rate which is regulated by said first factor which depends on thefilling degree of said one output queue.
 3. A method according to claim1, characterized in that the transmission network is an asynchronoustransfer mode network, and that said data packets, transferred at a bitrate depending on available network capacity, are formed by availablebit rate traffic.
 4. method of transferring data packets between aplurality of network elements in a teletransmission network, comprising:transferring at least some data packets at a bit rate which depends onavailable network capacity; positioning packets, when arriving at anetwork element, in a plurality of input queues; transferring said atleast some data packets from the input queues to at least one outputqueue provided specifically for said at least some packets at a bit ratewhich is regulated in several steps by a first factor which depends onthe filling degree of said at least one output queue; characterized inthat said first factor for the at least one output queue is currentlycalculated on the basis of the mean value of the filling degree of theat least one output queue during a preceding time span.
 5. A method oftransferring data packets between a plurality of network elements in ateletransmission network, comprising: transferring at least some datapackets at a bit rate which depends on available network capacity;positioning packets, when arriving at a network element, in a pluralityof input queues; transferring said at least some data packets from theinput queues to at least one output queue provided specifically for saidat least some packets at a bit rate which is regulated in several stepsby a first factor which depends on the filling degree of said at leastone output queue; characterized in that the bit rate, at which said atleast some data packets are transferred from the input queues to the atleast one output queue is moreover regulated by a second factor which isindicative of a mutual weighting of the input queues.
 6. A methodaccording to claim 5, wherein the data packtes are transferred via aplurality of virtual connections set up in the network, characterized inthat an input queue is allocated to each virtual connection which passesthe network element concerned.
 7. A method according to claim 6,characterized in that said another second factor for each input queue isa predetermined weight factor for the connection associated with theinput queue with respect to the other connections which are set up inthe network.
 8. A network element for use in a teletransmission network,wherein data packets are transferred between a plurality of networkelements at a bit rate which depends on available network capacity, saidnetwork element comprising; a plurality of input queues in which datapackets, when arriving at a network element, are positioned; one or moreoutput queues to which the data packets may be transferred from theinput queues; and means for transferring the data packets from the inputqueues to the one or more output queues, characterized in that at leastone particular output queue is provided specifically for said at leastsome data packets and that said means are arranged to transfer said atleast some data packets to said at least one particular output queue ata bit rate which may be regulated in several steps by a first factorwhich depends on the filling degree of said at least one particularoutput queue; means for completely interrupting the transfer of datapackets from the input queues to said at least one particular outputqueue independent of said regulation of the bit rate if said at leastone particular output queue concerned is full.
 9. A network elementaccording to claim 8, characterized in that it is adapted for use in anasynchronous transfer mode network, wherein said data packets,transferred at a bit rate depending on the available network capacity,are formed by available bit rate traffic.
 10. A network element for usein a teletransmission network, wherein data packets are transferredbetween a plurality of network elements at a bit rate which depends onavailable network capacity, said network element comprising: a pluralityof input queues in which data packets, when arriving at a networkelement, are positioned; one or more output queues to which the datapackets may be transferred from the input queues; and means fortransferring the data packets from the input queues to the one or moreoutput queues, characterized in that at least one particular outputqueue is provided specifically for at least some data packets and thatsaid means are arranged to transfer said at least some data packets tosaid at least one particular output queue at a bit rate which may beregulated in several steps by a first factor which depends on thefilling degree of said at least one particular output queue,characterized in that each input queue corresponds to a virtualconnection in the network.
 11. A network element for use in ateletransmission network, wherein data packets are transferred between aplurality of network elements at a bit rate which depends on availablenetwork capacity, said network element comprising: a plurality of inputqueues in which data packets, when arriving at a network element, arepositioned; one or more output queues to which the data packets may betransferred from the input queues; and means for transferring the datapackets from the input queues to the one or more output queues,characterized in that at least one particular output queue is providedspecifically for at least some data packets and that said means arearranged to transfer said at least some data packets to said at leastone particular output queue at a bit rate which may be regulated inseveral steps by a first factor which depends on the filling degree ofsaid at least one particular output queue, characterized in that itmoreover comprises means for regulating the bit rate, at which the datapackets are transferred from the input queues to the output queue, by asecond factor which is indicative of a mutual weighting of the inputqueues.
 12. A network element for use in a teletransmission network,wherein data packets are transferred between a plurality of networkelements at a bit rate which depends on available network capacity, saidnetwork element comprising: a plurality of input queues in which datapackets, when arriving at a network element, are positioned; one or moreoutput queues to which the packets may be transferred from the inputqueues; and means for transferring the data packets from the inputqueues to the one or more output queues, characterized in that at leastone particular output queue is provided specifically for at least somedata packets and that said means are arranged to transfer said at leastsome data packets to said at least one particular output queue at a bitrate which may be regulated in several steps by a first factor whichdepends on the filling degree of said at least one particular outputqueue, said network element including a switching unit, a plurality ofinput modules, and a plurality of output modules, characterized in thatsaid input queues are positioned on the output modules.
 13. A networkelement according to claim 12, characterized in that said at least oneparticular output queue is positioned in the switching unit.
 14. Anetwork element according to claim 12, characterized by a plurality ofoutput queues which at least corresponds to the plurality of outputmodules, said plurality of output queues being positioned with at leastone output queue on each output module.